MINOR MISSION SPOILERS
Buy the Collectibles Locator for your map like the other solutions say, but to make this achievement come most easily, I suggest traveling to each collectible location with the Specter (Hover Bike), which you have the option of unlocking on the mission "My Name is Cyrus Temple".
During that mission you choose two prototypes to sign off on (Specter, improved Tank, and satchel charges).
Make sure you choose the Specter as one of the two because it is as fast as any flying vehicle, allowing you to get around the various locations like rooftops and caves quickly and easily, but also durable and small enough to make it an easy vehicle to land, compared to the VTOL or a chopper. Using the Specter and the Collectibles locator map, this achievement takes about 45 minutes to an hour.
There are 80 Collectibles to obtain (20 blow-up dolls 20 cash palettes, 20 drug palettes, and 20 photo ops) and most of them are easy to get to. Only a few times did I have to even get off the Specter because you can drive over them to pick them up.
A few of them are inside caves or buildings, and others are on roofs, but believe me, there is no simpler solution than traveling via the Specter.
It's a bit tedious no matter what you do, but trying this achievement without both the map and at least some flying vehicle (again, preferably the Specter) is gaming suicide.